1. Field of the Invention
This invention relates generally to methods of making crystals, and more particularly to the growth and specification of crystals achieving highly reproducible performance for surface acoustic wave devices (“SAW”).
2. Description of the Related Art
Suppliers of Lithium Niobate (LN) typically differentiate between LN for optical applications and that for surface acoustic wave (SAW) applications. Because crystals used for optical applications typically need better uniformity and lower optical loss, optical LN crystals typically use higher purity starting materials, shorter length of crystals, lower solidified fractions of melt volume to crystal volume for each growth run, and a more stringent inspection of the as-grown crystal. This lower efficiency of optical material growth translates into higher cost for a finished optical wafer as compared to SAW wafers.
SAW filters manufactured from LN substrates are utilized in many applications, for example in TV receivers and mobile communication base stations. The purpose of such a filter generally is to pass the signal frequencies in the pass band, and significantly attenuate those in the stop band. High performance filters often have requirements of attenuating certain frequencies to less than −65 dB as compared to the pass band frequencies.
A shift in substrate SAW velocity is detrimental to the filter manufacturing yield as it shifts the filter's frequency response. SAW wafer customers therefore require good velocity uniformity not only within a wafer, but also from wafer to wafer and from wafers cut out from different crystal over long time periods. At the same time, the SAW substrates need to be very cost effective to address the consumer's expectations of falling prices for electronics equipment. For the crystal producer, growing LN crystals from the melt, this requires conversion of the highest amount of melt into useful crystal material while at the same time controlling the SAW velocity within a narrow range. It has been shown that the major factor in SAW velocity variation for LN is a deviation in composition from congruency (K. Yamada, H. Tekemura, Y. Inoue, T. Omi, and S. Matsumura, “Effect of Li/Nb Ratio on the SAW Velocity of 128° Y-X LiNbO3 Wafers,” Japanese Journal of Applied Physics 26-2(Supplement 26-2), 219-222 (1987)). The SAW velocity deviation from the center value due to compositional variation for the commonly used LN 128° wafers is 10900 ppm per mol % of lithium oxide. Other orientations of lithium niobate, or crystals that may be grown from melts close to congruency also will show a shift of SAW velocity with compositional changes.
Uniform crystals can be grown from the congruent composition, producing a crystal of the same composition as that of the melt. Early work on LN growth has determined the congruent composition to be xc=[Li]/([Li]+[Nb])=48.6% (J. R. Carruthers, G. E. Peterson, M. Grasso, and P. M. Bridenbaugh, “Nonstoichiometry and crystal growth of Lithium niobate,” J. Appl. Phys. 42, 1846-1851 (1971)). This value has been refined subsequently, first by Abrahams to 48.45%, and finally by Bordui to 48.38%. (S. C. Abrahams and P. Marsh, “Defect structure dependence on composition in lithium niobate,” Acta Crystallogr., Sect. B: Struct. Sci. 42, 61 (1986), and P. F. Bordui, R. G. Norwood, and J. L. Nightingale, “Composition for growth of homogeneous lithium niobate crystals,” U.S. Pat. No. 5,310,448 (1994)).
LN substrate users would like wafer-to-wafer variability to be contained within narrow limits. One of the accepted methods for specifying crystal composition is by specifying a range for the Curie temperature with the correlation between composition and Curie temperature as described by Bordui et. al. (P. F. Bordui, R. G. Norwood, D. H. Jundt, M. M. Fejer, J. Appl. Phys. 1992, 71(2), 875-879). For optical grade LN, leading suppliers adhere to very tight specifications on Curie temperature. For example, Crystal Technology, Inc. states that optical wafers are “congruent within 0.02 mol %” which equates to a specification tolerance of ±0.7° C. for the Curie temperature. SAES Getters (Milano, Italy) also states a specification range of ±0.7° C., even though at a slightly different center value (1140 as opposed to 1142.3° C. for Crystal Technology, Inc.).
Other suppliers of optical LN typically have a less demanding specification. For example, Isowave (Dover, N.J.) cites compositional uniformity (within a wafer) of 0.02 mol % but does not call out a wafer to wafer repeatability. For acoustic, or SAW grade material, wafer price is a primary concern and the wafer manufacturer lowers cost as compared to optical growth by achieving a high melt solidified fraction. For SAW grade material, such values exceed 60% typically being above 85% (P. F. Bordui, et.al, U.S. Pat. No. 5,310,448 (1994)), incorporated herein by reference. Because of this high solidified fraction, the compositional control, and the specification for Curie temperature need to be relaxed. Most supplier's Curie specification for SAW wafers has a window of ±2° C. to ±3° C. (Yamaju Ceramics Co., Ltd; 1132±2° C., J&S Crystal Technology LTD. 1142±3° C., Crystal Technology, Inc. 1142.3±1.9° C.). When looking at those stated specification windows, one needs to be aware at how these are to be interpreted: The fact that all the leading suppliers state an absolute temperature indicates that the wafer to wafer (and crystal to crystal) variability is controlled within absolute values, even if the disparity in the stated numbers indicate large systematic differences in measurement methods or absolute temperature calibration.
Without additional information given in the specification, the specification is to be interpreted according to the relevant IEC standard (IEC 62276, Single crystal wafers for surface acoustic wave (SAW) device applications—Specifications and measuring methods Current specifications) in which the Curie range is specified to be no more than ±3° C. from the center value, and an AQL (acceptable quality level) of 2.5% is assumed for the listed defect groups collectively. Thickness specification, polish quality and cleanliness are quality classes that may also lead to some fallout, and one would expect that the crystal grower needs to control the Composition such that the Curie temperature exceeds the limit in no more than about 1% of wafers.
The first report on controlling weighing procedure for preparing the starting melt of LN is by O'Bryan (H. M. O'Bryan, P. K. Gallagher, and C. D. Brandle, “Congruent composition and Li-rich phase boundary of LiNbO3,” J. Am. Ceram. Soc. 68, 493-496 (1985)). The authors dried the 99.999% pure powders at 500° C. for 12 to 14 hours, then weighed each with a relative precision to the reacted LN of 4×10−5. The determination of composition was based on the assumption that the dried powders were pure compounds without any residual water or Li2O present. Bordui (P. F. Bordui, et al, J. Crystal Growth 113, 61-68 (1991)) also employed drying to account for moisture and additionally performed thermal ionization mass spectrometry to determine the atomic weight of lithium.
The corrections were then applied to the weighing recipe to produce a charge that was estimated to be accurate within ±0.01 mol %. In U.S. Pat. No. 5,320,448, the same authors describe the drying method to take place at 500° C. in air for 10 minutes before removing the powder sample and weighing it after another 10 minutes have elapsed. Moisture correction factors of 1.0058 for the carbonate and 1.0002 for the pentoxide were established. The prior art therefore takes into account some possible corrections that are needed and performs tests on the powders to estimate the necessary corrections.
However, it is not certain that these corrections account for all possible contaminants/or powder degradations. It is possible that some water is more tightly bound and not liberated at the 500° C. temperature. It is possible that organic contaminants are present in the powders. These contaminants are typically not tested for in the purity analysis and may be present in the tens of ppm amounts even for 99.999% pure powders. It is possible that a significant fraction of lithium carbonate dissociates into Li2O and CO2 with one or both of those products volatilizing during the powder drying test.
There are other materials that melt congruently, for example Langasite (La3Ga5SiO14) and similar compounds, but have to be grown with excess of one of the initial components, typically Ga2O3to compensate for volatilization. The non-negligible volatilization makes it difficult to grow homogeneous crystals at large melt conversion fractions. Large non-uniformities have been observed in this material (S. Uda, A. Bungo, and C. Jian, “Growth of 3-inch Langasite Single Crystal and Its Application to Substrate for Surface Acoustic Wave Filters,” Japanese Journal of Applied Physics 38(Part 1, No. 9B), p 5516 (1999)).